This invention relates generally to an electronic ballast suitable for igniting a ceramic metal halide lamp and, more particularly, to a electronic ballast that utilizes high frequency short durations pulses to ignite a ceramic metal halide lamp.
Ceramic metal halide (CMH) lamps provide several advantages over conventional metal halide lamps including a more uniform color spectrum. A CMH lamp includes a discharge tube defining an interior region. Upon application of sufficient voltage across a pair of electrodes positioned within the interior region of the discharge tube, a high pressure arc is ignited in the discharge tube interior region.
However, certain operating characteristics of CMH lamps impose particular requirements and restrictions in the design of a ballast circuit to ignite and maintain an arc in a discharge tube of a CMH lamp. CMH lamps operate at high temperature and pressure. The ignition or break-over voltage required to ignite a CMH lamp discharge tube increases with increasing lamp temperature. A CMH lamp operated for a long period may have a discharge tube temperature exceeding 1000 degrees Celsius (xc2x0 C.). A typical CMH discharge tube having a break-over voltage of around 1.5 kilovolts (kV) at room temperature can have a break-over voltage of 15-20 kV when the discharge tube temperature increases to 1100 degrees xc2x0 C. Unfortunately, application of break-over voltages above 5 kV may result in failure of the components of the ballast circuit. For example, a momentary drop in power supplied by an AC power line may cause the discharge arc to be extinguished during operation of the CMH lamp. It is desired to reignite the arc in the discharge tube as soon as possible to provide illumination. However, if under such a hot restart condition the ballast circuit attempts to generate an output voltage greater than 5 kV because the break-over voltage exceeds 5 kV, damage to the ballast circuit will likely result. Thus, the ballast circuit must allow the discharge tube to cool sufficiently so that the break-over voltage drops from 15 kV (or more) to below 5 kV and simultaneously must limit the output voltage applied to the discharge tube to 5 kV.
An additional concern of the design of a ballast for a CMH lamp is frequency. The CMH lamp comprises a tube with ceramic material in the form of salts disposed in the tube interior region. Undesirable bowing of the discharge arc is caused by excitation of the discharge tube, application of power to the discharge tube at an acoustic node. For a typical 20 watt (W) CMH lamp, the resonant nodes may range from audible frequencies to 300 kiloHertz (kHz). Many other resonant nodes may exist but the upper limit usually is less than 1 megaHertz (MHz). Fortunately, at frequencies above 1 MHz, the effect of the acoustic resonant frequency is negligible on the discharge tube arc. Thus, it is preferable that the ballast circuit drive the discharge tube at a frequency above 1 MHz.
Because of Federal Communications Commission (FCC) regulations regarding electromagnetic interference (EMI) emissions, it is desirable to excite the lamp at frequencies between 2.5 to 3.0 MHz as higher magnitudes of emissions are permitted by the FCC within this frequency range xe2x80x9cchimney.xe2x80x9d
The electronic ballast should also provide protection against application of excess voltage under a hot restart condition of the discharge tube so as not to damage the ballast circuit components and allow the discharge tube to cool.
An electronic ballast for igniting an arc within a discharge tube of a ceramic metal halide (CMH) lamp. The electronic ballast includes a hot restart protection circuit to avoid application of an undesirably high voltage to the discharge tube during a hot restart situation. In one preferred embodiment, the ballast generates a series of short duration voltage pulses having a duration of 50 microseconds (xcexcs) and are applied at rate of approximately 1 pulse/second. Each 50 xcexcs pulse is comprised of a pulse train or a plurality of pulses having a peak voltage amplitude of between 2.0 to 2.5 kV (4.0 to 5.0 kV peak to peak).
The ballast periodically pulses the CMH lamp discharge tube approximately once per second until it ignites, that is, an arc is generated. The limitation on voltage amplitude (2.0 kV peak) of the voltage pulses prevents damage to the inverter circuit of the ballast from overvoltage application when the lamp is hot. The time between the 50 xcexcs voltage pulses of just less than 1 second keeps the inverter circuitry components from overheating.
These and other objects, advantages, and features of an exemplary embodiment of the present invention are described in detail in conjunction with the accompanying drawings.